[tex]n-\ number\ of\ nickels\\
q-\ number\ of\ quarters\\\\
n+q=12\ \ \ \ | n=12-q\\
0,05(n+12)=0,25q\\\\
0,05n+0,6=0,25q\ \ \ | substitute\ n=12-q \\\\
0,05(12-q)+0,6=0,25q\\\\
0,6-0,05q+0,6=0,25q\\\\
1,2-0,05q=0,25q\ \ \ | add\ 0,05q\\\\
1,2=0,3q\ \ \ | divide\ by\ 1,2\\\\
q=4\\\\
He\ has\ 4\ quarters.[/tex]